1. Field of the Invention
The present invention relates to a magnetoresistance effect sensor used for a magnetic head and the like.
2. Description of the Related Art
In general, information recorded on a magnetic recording medium is read by moving a read magnetic head using a coil relative to the magnetic recording medium and detecting a voltage induced in the coil by electromagnetic induction caused by the relative movement of the coil. It is also known that a magnetoresistance type head is used for an information reading operation [IEEE MAG-7, 150, (1971) ]. This magnetoresistance type head uses the phenomenon that the electric resistance of a ferromagnetic material of a certain type changes in accordance with the strength of an external magnetic field. This type of head is used as a high-sensitivity head for a magnetic recording medium. With the recent tendency to reduce the size of a magnetic recording medium while increasing its capacity, the speed of the read magnetic head relative to the magnetic recording medium is decreased. Therefore, demand for a magnetoresistance type head (to be referred to as an MR head hereinafter) capable of extracting a large output at a low relative speed is high.
MR elements used for MR heads include a single-layer MR element and a multilayer MR element which exhibits a larger magnetoresistance change than the single-layer MR film. The following types of multilayer MR elements are available. The first type of multilayer MR element is designed to exhibit a large magnetoresistance change by using the difference between the different coercive forces of two types of magnetic films which are arranged such that the magnetization of one film is antiparallel to that of the other film [The Journal of the Institute of Applied Magnetics of Japan Vol. 15, No. 5813 (1991)] (non-exchange coupled type).
The second type of multilayer MR element has a multilayer film constituted by two ferromagnetic films sandwiching a nonmagnetic film. In this element, an exchange bias based on an antiferromagnetic film is applied onto one ferromagnetic film of the multilayer film to fix its magnetization, and the magnetization of the other ferromagnetic film is inverted by an external magnetic field. As a result, the directions of magnetization of the two ferromagnetic films sandwiching the nonmagnetic film become parallel or antiparallel to each other, exhibiting a large magnetoresistance change [Phys. Rev. B., Vol. 4580, (1992)]; [J. Appl, Phys., Vol. 69, 4774 (1991)] (spin valve type).
In the third type of multilayer MR film, an antiparallel state of magnetization is obtained by a current magnetic field formed by a sense current, and a large magnetoresistance change occurs due to the difference between the antiparallel state and a parallel state of magnetization obtained by a signal magnetic field [IEEE TRANSACTION ON MAGNETICS Vol. MAG-20, No. 5,863 (1984)] (depth current scheme).
When an MR head is actually used, two types of bias magnetic fields must be applied to the MR element. One bias magnetic field is applied to the MR element in a direction perpendicular to the sense current flowing in the element and is generally called a transverse bias. The transverse bias is a magnetic field used to achieve a state wherein the magnitude of an external signal becomes proportional to that of a detected signal, i.e., a so-called operating point.
The other bias magnetic field is applied to the MR element in a direction parallel to the sense current and is generally called a longitudinal bias. The longitudinal bias serves to suppress Barkhausen noise which is generated due to the multi-domain characteristics of the MR element.
As methods of applying such a longitudinal bias, methods of using a magnetized ferromagnetic film have been proposed. For example, U.S. Pat. No. 3,840,898 discloses a method of applying a longitudinal bias to an MR layer, in which the MR layer is arranged to be adjacent to a magnetized hard film through a thin insulating film. According to this method, by selecting a magnetizing direction, a longitudinal bias or a transverse bias can be selectively applied, or a bias can be applied even in a direction therebetween. In addition, U.S. Pat. No. 4,103,315 discloses that a uniform longitudinal bias is generated in an MR layer upon exchange coupling between an antiferromagnetic film and a ferromagnetic film. It is also reported in JOURNAL OF APPLIED PHYSICS Vol. 52, 2474 (1981) that when an FeMn alloy film is used as an antiferromagnetic film, a longitudinal bias is applied to the MR layer.
On the other hand, it is preferable that a relatively weak longitudinal bias be applied to a central portion of an MR layer, i.e., a so-called active region, as compared with a bias applied to its end portion. This is because an excessive longitudinal bias magnetic field in a signal magnetic field causes a deterioration in sensitivity. Magnetic Recording Research Institute MR86-37 of the Institute of Electronics and Communication Engineers discloses a structure in which a high-coercive-force film as a magnetization stabilizing film is formed on only an end region of an MR layer. In this structure, a magnetized CoPt film is arranged on an end portion of the MR layer of a yoke type MR head to apply a longitudinal bias. In addition, it is reported in IEEE TRANS. MAG-25, 3692 (1989) that even if an FeMn film is formed on only an end portion of an MR layer, a longitudinal bias is applied to the active region of the PG,6 MR layer.
As described above, several methods of applying a longitudinal bias to an MR element have been proposed. However, if these methods are applied to a magnetic head for a hard disk drive, the following problems are posed.
When a magnetization stabilizing film is formed on the entire surface of an MR layer, a strong bias magnetic field is applied to the active region, resulting in a deterioration in sensitivity. In contrast to this, when a magnetization stabilizing film is formed on only an end portion of an MR layer, the following three problems are posed. First, when a magnetization stabilizing film is a high-coercive-force film, as shown in FIG. 1, positive or negative magnetization occurs in an end face 12 crossing the direction of magnetization of a high-coercive-force film 11 patterned on an MR layer 10, and the resulting leakage magnetic field adversely affects the MR layer 10, thus interfering with the stabilization of magnetization of the MR layer. Second, when a magnetization stabilizing film is formed on only an end portion of an MR layer by patterning, a film having a thickness of several tens of nanometers is subjected to ion milling. In patterning, therefore, the MR layer may be damaged. That is, the formation of such a film is difficult as a process. Third, assume that a high-coercive-force film having a thickness of about 20 nm or more is formed on only an end portion of an MR layer. In this case, if an MR layer having a thickness of about 30 nm is formed on the high-coercive-force film, conspicuous stepped portions are formed on the MR layer owing to the influence of the underlying layer. This causes a deterioration in the characteristics of the MR layer. In such a case, therefore, a high-coercive-force film is preferably formed on an MR layer. However, since an NiFe film as an MR layer promotes the c-axis orientation of a Co-based film as a high-coercive-force film, it is difficult to apply a longitudinal bias magnetic field.